射頻電漿濺鍍氮化鋁在有機薄膜電晶體閘極絕緣層之應用

Abstract

本論文中，低溫(25℃-250℃)濺鍍氮化鋁(AlN)薄膜被沉積當作有機薄膜電晶體(OTFTs)上的閘極絕緣層。不同於傳統的高介電常數的材料，氮化鋁擁有低的閘極漏電以及高的疏水性。在實驗中發現氮化鋁的表面能比五環素(pentacene)還低而且近似於用自組裝單層膜(SAM)處理過後的介電層。在實驗中，嘗試調整氮化鋁的製程條件，我們發現當製程溫度降低時，氮化鋁可獲得平整的表面(表面粗糙度小0.2nm)以及很低的閘極漏電(在1MV/cm，漏電流小於1×10-9A/cm2)。並且觀察到崩潰電場大於5MV/cm以及量測到的氮化鋁介電常數為7。除了降低溫度外，當增加氮氣流率比時，氮化鋁漏電也獲得抑制。更近一步分析漏電行為，發現氮化鋁在低電場時，由歐姆(Ohmic)定律主導。在高電場時，由普爾-夫倫克爾(Poole-Frenkel)過程來主導。因此當增加氮氣流率時，氮相關之缺陷可能獲得補償，漏電因此而降低。 根據最佳製程條件，我們實現了低電壓操作的氮化鋁有機薄膜電晶體。當氮化鋁的厚度降低到70nm以下，氮化鋁有機薄膜電晶體可以被操作在5伏特以下。並且可以獲得高的電流開/關比值(大於10^6)以及高的載子移動率(約1.67cm2/V-sec)。而且也可得到極低的次臨界擺幅(約130mV/decadec)和臨界電壓(約-1.5伏特)。然而由於氮化鋁擁有低的表面能，可能導致低的介面捕獲電荷密度和極小的次臨界擺幅。這些顯著的電晶體特性論證了氮化鋁有機薄膜電晶體擁有極大的潛力，可以應用在低操作電壓以及快速開關的有機電晶體上。

In this thesis, the low-temperature (25℃-250℃) sputtered aluminum nitride (AlN) film was deposited as the gate insulator in organic thin film transistors (OTFTs). In contrary to the conventional high-k dielectrics, the AlN film has a very low dielectric leakage and hydrophobic. The surface free energy is lower than the pentacene film and similar to the self-assembled monolayer (SAM) treated dielectrics. By adjusting the AlN sputtering process, it was found that the smooth (surface roughness <0.2nm) and very low leakage (less than 1×10-9A/cm2 at 1MV/cm) AlN film was obtained by lowering down the process temperature. The dielectric breakdown field is larger than 5MV/cm and the relative electrical permittivity is 7 in the AlN film. With the increasing of nitrogen gas flow rate, the dielectric leakage of AlN film is suppressed. Moreover, the leakage-current in AlN film obeys the Ohmic-conduction and the Poole-Frenkel transport in low and high electric field, respectively. It is suggested that the nitrogen-related defects are compensated, with the increasing of nitrogen flow rate in sputtering process, thus the leakage is decreased. Accordingly to the optimized sputtering conditions, the low-voltage AlN-OTFTs is realized. With the thin AlN gate-dielectric (thickness less than 70nm), the AlN-OTFTs are operated at a very low voltage (less than 5V). It is founded that the high on/off current ratio more than 10^6, and the high mobility about 1.67cm2/V-sec are achievable. The lowest subthreshold swing is 130mV/decade and the threshold voltage is -1.5V. The low interface-trap-density and the steep subthreshold swing for AlN-OTFTs may be attributed to the low surface energy of the AlN film. The remarkable transistor properties demonstrate that the AlN-OTFT has potential application in low-voltage and rapid-switching organic transistors.